The Initiation and Development of Secondary Xylem in Axillary Branches of Populus deltoides

The initiation of secondary xylem in elongating axillary branchesof Populus deltoides Bartr. ex Marsh. is independent of thatin the main stem. Although secondary xylem differentiates acropetallyin the main stem, it does not differentiate from the stem intothe axillary branch. Secondary xylem is usually initiated ininternode 4 (occasionally 3) of the axillary branch, and fromthis site it develops both acropetally in the elongating branchand basipetally toward the main stem. Secondary vessel differentiationalways precedes fibre differentiation. Although secondary xylemdifferentiates in internodes that have ceased elongation, itdifferentiates first in traces of the vascular cylinder servingrapidly expanding and maturing foliage leaves. As younger leaveson the branch expand and mature, secondary xylem differentiatesin their traces eventually producing a complete secondary vascularcylinder. Scale leaves do not initiate secondary xylem independentlyin their traces; they are activated by adjacent traces in thevascular cylinder serving foliage leaves. Once established,the primary-secondary vascular transition zone advances acropetallyin a branch just as it does in the main stem. Populus deltoides Bartr. ex Marsh., cottonwood, axillary branches, secondary xylem, plastochron index, post-dormancy development, xylem.     

Root architecture of riparian trees: river cut-banks provide natural hydraulic excavation, revealing that cottonwoods are facultative phreatophytes

Plant root architecture reveals the sources of water and nutrients but tree root systems are large and difficult to analyze. With riparian (floodplain) trees, river cut-banks provide natural hydraulic excavation of root systems and this presents a unique study opportunity. Subsequently, we developed the ‘Cut-bank Root Method’, a simple, quantitative approach for analyzing the distribution of coarse roots, based on analyses of photographs of river cut-banks. These reveal the vertical extent of roots and median root depths (R_d). We applied this method along six rivers draining the Canadian Rocky Mountains and observed tenfold difference in R_d. The floodplain forests were dominated by cottonwoods and from mountain to prairie zones we observed progressively deeper roots of Populus trichocarpa (black cottonwood, R_d ~ 0.3 m), P. balsamifera (balsam poplar), P. angustifolia (narrowleaf cottonwood), and P. deltoides (prairie cottonwood, R_d ~ 0.9 m), which had R_d similar to P. fremontii (Fremont cottonwood) in Nevada, USA. Roots were shallower for co-occurring facultative riparian trees, with R_d ~ 0.1 m for P. tremuloides (trembling aspen) and Picea glauca (white spruce). Across the Canadian sites, R_d for cottonwoods were strongly associated with a growth season moisture index (May through September precipitation—potential evapotranspiration; R² = 0.97, P < 0.001). Thus, in wetter climates, riparian cottonwoods were shallow-rooted and would be more dependent upon rain than stream flow. Conversely, in the drier semi-arid regions the cottonwoods were phreatophytic, with deeper root systems in the capillary fringe above the alluvial ground-water table. These phreatophytic cottonwoods would be highly dependent upon stream flow and vulnerable to declining river flows due to river regulation or climate change.     

Direct seeding for riparian tree re-vegetation: Small-scale field study of seeding methods and irrigation techniques

Restoration of wetland and associated ecosystems is a major goal of land management agencies throughout the world. On the lower Colorado River, creation of riparian forests is planned to mitigate riparian habitat degradation by historic land-use conversions and river management. Current restoration practices use propagated plant stock. If direct seeding can be implemented, genetic and structural diversity could be enhanced at restoration sites even while reducing costs compared to vegetative propagation methods. A small-scale field study was implemented in Cibola, Arizona, to determine the effectiveness of direct seeding of Fremont cottonwood (Populus fremontii), Goodding's willow (Salix gooddingii), and coyote willow (S. exigua). For the first growing season, establishment of Fremont cottonwood averaged 7% of pure live seed rates for all treatments combined, whereas establishment of willows was less than 1%. Volunteer species were abundant, with grasses dominating cover and biomass after one growing season. Saltcedar (Tamarix ramosissima) established in abundance, but showed lower growth rates than Fremont cottonwood during the first growing season. Monitoring for three growing seasons indicated higher growth rates and survival of Fremont cottonwood compared to all volunteer species. Study results indicated that direct seeding of Fremont cottonwood is likely to be an efficient method for tree re-vegetation. Additional studies are required for willow species to determine if establishment from seed can be increased through enhanced weed control and elimination of Fremont cottonwood from the seed mix.     

The Relationship Between Size of Shoot Apices and Morphological Features of Mature Leaves and Stems of Four Species of Angiosperm Trees

Shoot apex, leaf and stem growth parameters for four speciesof deciduous trees were measured. Only in elm was there a correlationbetween the size of shoot apical meristems and mature leaves.In ash, basswood and cottonwood there was no significant differencebetween size of shoot apices of sucker and canopy branches,despite significant differences in lamina size. In the suckerbranches of all species studied there occurred an early, lateralexpansion of the subapical region of the shoot apical meristem.This correlated well with the greater diameter of stem and pithregions of sucker branches. In addition, the season's annualring of xylem was greater in basswood, cottonwood and elm. Diametersof vessel elements were greater in sucker than canopy branchesin three of the four species. Total branch and internode andnumber of nodes per branch were significantly greater for suckerbranches than canopy growth of all species studied. A hypothesis is proposed to explain the development of the verylarge surface area of leaves on sucker branches. This hypothesisis based on the position of sucker branches in relation to theroot system and involves differences in water stress known tobe present in all plants.Copyright 1993, 1999 Academic Press Sucker leaves, canopy leaves, Fraxìnus pennsylvanica Marsh, green ash, Ulmus amerìcana L., American elm, Populus deltoides Marsh, cottonwood, Tilia americana, basswood     

Regeneration and invasion of cottonwood riparian forest following wildfire

Populus deltoides is considered to be a weak resprouter and highly susceptible to wildfire, but few post‐wildfire studies have tracked P. deltoides response and resprouting within the Great Plains of North America. Following a wildfire in southwestern Kansas, U.S.A., we surveyed burned and unburned areas of a cottonwood riparian forest along the Cimarron River that included a major understory invader, tamarisk (Tamarix ramosissima Ledeb.). We tested the following hypotheses, which are consistent with the current understanding of P. deltoides response to wildfire in the Great Plains: (1) regeneration of P. deltoides will be low in areas burned by the wildfire; (2) the number of dead P. deltoides individuals will be greater in the wildfire than unburned areas; and (3) tamarisk regeneration will be higher than P. deltoides regeneration in the wildfire areas because tamarisk is considered a stronger resprouter. We found evidence contrary to two of our hypotheses 3 years following the wildfire. (1) P. deltoides regeneration was high following the wildfire, averaging 692 individuals/ha. (2) The number of dead mature cottonwood trees was greater in wildfire plots than in unburned plots. (3) There was more P. deltoides regeneration than tamarisk regeneration following wildfire. These findings, which diverge from the majority of studies examining P. deltoides regeneration in the Great Plains, suggest that differing local environmental and forest stand conditions, coupled with the timing and intensity of the fire, could be important determinants of riparian forest species' responses to wildfire.     

The Proportion of Three Foundation Plant Species and Their Genotypes Influence an Arthropod Community: Restoration Implications for the Endangered Southwestern Willow Flycatcher

As part of a restoration project, multiple genotypes of two tree species, Fremont cottonwood (Populus fremontii) and Goodding's willow (Salix gooddingii), and one shrub species, Coyote willow (S. exigua), were experimentally planted in different proportions at the Palo Verde Ecological Reserve near Blythe, California, U.S.A. These common woody plant species are important to the endangered southwestern willow flycatcher, providing perch, nesting, and foraging habitat. We conducted this study to evaluate plant species proportion and plant genotype effects on the arthropod community, the prey base for the endangered southwestern willow flycatcher. Three patterns emerged. First, plant species proportions were important; the arthropod community had the greatest richness and diversity (H′) when Goodding's willow proportion was high and Fremont cottonwood proportion was lower; that is, fewer Fremont cottonwoods are required to positively affect overall arthropod diversity. Second, we found significant genotypic effects, for all three plant species, on arthropod species accumulation. Third, while both planting proportion and genotype effects were significant, we found that the effect of planting proportion on arthropod richness was about twice as large as the effect of plant genotype. This shows that both plant species proportions and genotype should be utilized in restoration projects to maximize habitat heterogeneity and arthropod richness. Similar studies can determine which planting proportion and specific genotypes may result in a more favorable arthropod prey base for the southwestern willow flycatcher and other species of concern. Greater attention to planting design and genotype can result in significant gains in diversity at little or no additional project cost.     

Genetic analysis of admixture and patterns of introgression in foundation cottonwood trees (Salicaceae) in southwestern Colorado,USA

Cottonwoods are well known as foundation riparian trees that support diverse communities and drive ecosystem processes. Although hybridization naturally occurs when the distributions of two or more cottonwood species overlap, few cottonwood hybrid zones have been genetically characterized. We use genetic and genomic analyses to characterize patterns of admixture and introgression for a newly described hybrid zone at the intersection of three species (Populus L. Salicaceae—Populus deltoides, Populus fremontii, and Populus angustifolia) in southwestern Colorado, USA. Analysis of nuclear and chloroplast microsatellite marker data detected substantial genetic variation among individuals, revealing that (1) hybridization is occurring between two, not three, species (P. deltoides and P. angustifolia); (2) gene flow is bidirectional; (3) hybrids are not abundant (admixture detected in only 34 of 270 trees), with most being early-generation F₁ hybrids; (4) cytonuclear disequilibria exists and F₁ hybrids tend to retain P. deltoides—like chloroplasts; and (5) roughly 30 % of the nuclear markers deviated from a neutral pattern of introgression, suggesting that selection may play a role in shaping the genetic structure of the hybrid zone in this region. Overall, our results show that despite strong selection maintaining species divergence, transfer of allelic variants across species boundaries can occur. Our study assesses the fine-scale genetic structure of hybridization between P. angustifolia and P. deltoides and lays the foundation for examining how geographic differences in hybrid zone dynamics arise and may influence subsequent ecological and evolutionary processes.     

Arthropod communities on hybrid and parental cottonwoods are phylogenetically structured by tree type: Implications for conservation of biodiversity in plant hybrid zones

Although hybridization in plants has been recognized as an important pathway in plant speciation, it may also affect the ecology and evolution of associated communities. Cottonwood species (Populus angustifolia and P. fremontii) and their naturally occurring hybrids are known to support different plant, animal, and microbial communities, but no studies have examined community structure within the context of phylogenetic history. Using a community composed of 199 arthropod species, we tested for differences in arthropod phylogenetic patterns within and among hybrid and parental tree types in a common garden. Three major patterns emerged. (1) Phylogenetic diversity (PD) was significantly different between arthropod communities on hybrids and Fremont cottonwood when pooled by tree type. (2) Mean phylogenetic distance (MPD) and net relatedness index (NRI) indicated that communities on hybrid trees were significantly more phylogenetically overdispersed than communities on either parental tree type. (3) Community distance (D_pw) indicated that communities on hybrids were significantly different than parental species. Our results show that arthropod communities on parental and hybrid cottonwoods exhibit significantly different patterns of phylogenetic structure. This suggests that arthropod community assembly is driven, in part, by plant–arthropod interactions at the level of cottonwood tree type. We discuss potential hypotheses to explain the effect of plant genetic dissimilarity on arthropod phylogenetic community structure, including the role of competition and environmental filtering. Our findings suggest that cottonwood species and their hybrids function as evolutionarily significant units (ESUs) that affect the assembly and composition of associated arthropod communities and deserve high priority for conservation.     

Quantitative genetics of yield breeding forPopulus short rotation culture. III. Efficiency of indirect selection on tree geometry

Quantitative analysis of genetic covariances was used to identify the critical morphological components of wood productivity and to evaluate the efficiency of indirect selection for these components at the four levels of biological organization, (1) leaf, (2) branch, (3) main stem, and (4) whole-tree, in 4-yearPopulus deltoides ×P. simonii andP. deltoides ×P. nigra F₁ progeny. A total of 44 morphometric traits measured at the four organizational levels showed varying genetic associations with productivity, variations being dependent on traits, developmental processes (current terminal, sylleptics, and proleptics), and hybridization combinations. Most of the leaf and branch traits on the current terminal and/or sylleptic branches had higher genetic correlations with stem-wood volume than those on proleptics, which resulted in larger indirect selection responses in volume, especially in DxS progeny. Indirect clonal selection on leaf size and area, branching capacity, and branch angle at age 4 years was expected to generate 10–35% more genetic gain per year in 6-year volume than direct selection on 6-year volume in the DxS progeny. The efficiency of indirect selection on the numbers of different order branches and bifurcation ratio was greater than 1.0 relative to that for direct selection for stemwood volume in the D × N progeny. Under the pressure of artificial selection for superior volume production, with the proportion selected=15%, the two F₁ progeny populations exhibited distinct evolutionary divergence in tree geometry. The high-yielding D × S clones displayed a decurrent-like crown with strong apical dominance, whereas the crown ideotype for the high-yielding D × N clones was found to be excurrent-like and surrounded by dense foliage and branches.     

Ribosomal RNA genes structure in somePopulus spp. (Salicaceae) and their hybrids

The tandemly repeated multigene families encoding 18S and 25S rRNAs were studied at the restriction enzyme level inPopulus alba L.,Populus deltoides Bartr. exMarsh.,Populus trichocarpa Torr. & Gray and in the hybrids between the last two mentioned species. The analysis of single and double digestion with EcoRI, BamHI, XbaI, and SstI endonucleases showed the presence of single repetitive unit types of 12.25 and 11.75kb inP. alba andP. trichocarpa, respectively.P. deltoides showed two rDNA gene types having the same length (12.25Kb) but different nucleotide sequence in the IGS. The rDNAs genes ofP. deltoides andP. triochocarpa are inherited codominantly in their hybrids.     

Aluminum,Fe, Ca,Mg, K,Mn, Cu,Zn and P in above- and belowground biomass. I.Abies amabilis andTsuga mertensiana

In a mature mixed subalpine stand ofTsuga mertensiana andAbies amabilis, significantly higher Al levels were found in foliage, branch and root tissues ofT. mertensiana.Tsuga mertensiana had significant increases in Al, Ca and Mn levels with increasing foliage age. In current foliage,T. mertensiana had lower levels of Ca, similar levels of Mg and P, and higher levels of Mn thanA. amabilis. Both tree species had Cu and Fe present at higher levels in branch than foliage tissues. Fine roots had the highest concentrations of Al, Fe and Cu but the lowest Ca and Mn concentrations of all tissues analyzed. In the roots of both species, phloem tissues always had significantly higher Al levels than xylem. Fine roots (< 1 and 1–2 mm) ofT. mertensiana had higher Al levels than were found inA. amabilis. Roots greater than 2 mm in diameter exhibited no significant differences in Al levels in phloem or xylem tissue betweenA. amabilis andT. mertensiana. The two species show a clear difference in their ability to accumulate specific elements from the soil.     

A morphological and chemical study of Populus acuminata Rydberg

Evidence from morphology, flavonoid chemistry, and field observations suggests thatPopulus acuminata is of hybrid origin. The putative parents areP. angustifolia, the narrow leaf cottonwood, and deltoid leaved plants that are assigned toP. sargentii (P. deltoides var.occidentalis), P. fremontii, orP. wislizenii (P. fremontii var.wislizenii). Populus angustifolia exhibits a series of flavonol glycosides (kaempferol, quercetin, and myricetin) in its leaves. By contrast, the major leaf flavonoids of the broad leaved plants are flavone glycosides (apigenin and luteolin).Populus acuminata is intermediate between the suspected parents in morphological features. Additionally, the leaves of mostP. acuminata plants contain the exact summation of the flavonoid compounds characteristic of the putative parents. A diploid chromosome number of 2n = 38 was obtained for six plants, which confirms the one previous report for the species. Meiosis was regular in all cases. Correlated data indicate that the majority of plants ofP. acuminata represent F₁ hybrids and that complex hybridization is not common. Evidence from morphological and chemical studies is presented to show that in certain instances backcrossing to both parents has occurred. Results gathered in this study show thatP. ×andrewsii is undoubtedly “typical”P. acuminata, but the type specimen is from a sucker shoot, and thus has been interpreted as a backcross toP. sargentii. Populus acuminata var.rehderi is not considered worthy of taxonomic recognition.     

An Agrobacterium tumefaciens transformation protocol effective on a variety of cottonwood hybrids (genus Populus)

 We describe a protocol for Agrobacterium tumefaciens-mediated transformation of hybrid cottonwoods (Populus sections Tacamahaca Spach. and Aigeiros Duby). The protocol has allowed routine transformation of several economically important cottonwood hybrids (Populus trichocarpa Torr. & Gray×P. deltoides Bartr. ex. Marsh. and P. deltoides×P. nigra L.) that were previously difficult to transform. The procedure was applied to 11 different hybrid cottonwood genotypes and one P. deltoides genotype using kanamycin as the selection agent. Additional experiments showed a very strong interaction between auxin preculture and the effectiveness of various cytokinins for induction of shoot organogenesis. The data also demonstrated the superiority of Agrobacterium strain EHA105 over C58 and LBA4404 for T-DNA transfer based on transient assays with a reporter gene. Received: 16 June 1998 / Revision received: 5 February 1999 / Accepted: 14 April 1999     

Selection for genetics‐based architecture traits in a native cottonwood negatively affects invasive tamarisk in a restoration field trial

Climate change and competition from invasive species remain two important challenges in restoration. We examined the hypothesis that non‐native tamarisk (Tamarix spp.) reestablishment after aboveground removal is affected by genetics‐based architecture of native Fremont cottonwood (Populus fremontii) used in restoration. As cottonwood architecture (height, canopy width, number of stems, and trunk diameter) is, in part, determined by genetics, we predicted that trees from different provenances would exhibit different architecture, and mean annual maximum temperature transfer distance from the provenances would interact with the architecture to affect tamarisk. In a common garden in Chevelon, AZ, U.S.A. (elevation 1,496 m), with cottonwoods from provenances spanning its elevation distribution, we measured the performance of both cottonwoods and tamarisk. Several key findings emerged. On average, cottonwoods from higher elevations were (1) two times taller and wider, covered approximately 3.5 times more basal area, and were less shrubby in appearance, by exhibiting four times fewer number of stems than cottonwoods from lower elevations; (2) had 50% fewer tamarisk growing underneath, which were two times shorter and covered 6.5 times less basal area than tamarisk growing underneath cottonwoods of smaller stature; and (3) the number of cottonwood stems did not affect tamarisk growth, possibly because the negative relationship between cottonwood stems and basal area. In combination, these findings argue that cottonwood architecture is affected by local conditions that interact with genetics‐based architecture. These interactions can negatively affect the growth of reinvading tamarisk and enhance restoration success. Our study emphasizes the importance of incorporating genetic and environmental interactions of plants used in restoration.     

rDNA cloning and rapid hybrid identification inPopulus spp. (Salicaceae)

Ribosomal DNA genes fromP. deltoides have been cloned and specific sequences of the 25 S and 18 S rDNA region, labelled by digoxigenin, have been used to determine the rDNA structure ofPopulus tremula, P. fremontii, P. maximowiczii, P. yunnanensis, P. nigra, P. wislizenii, P. alba. The restriction maps of the coding region appeared to be similar among the examined species and with those ofP. deltoides andP. trichocarpa, reported in a previous paper. Inter- and intraspecific variation in rDNA repeat unit length have been revealed after EcoRI digestions. SstI and XbaI restriction sites have been found at different positions in the IGS of some species. The polymorphic fragments generated by SstI digestion allowed the identification of the hybrid origin of some genotypes. The number of rDNA genes in the genome ofP. deltoides has been estimated to be about 2 000 copies. Finally, the usefulness of these studies inPopulus spp. taxonomy and forestry genetics is discussed.Ribosomal RNA gene structure in somePopulus spp. (Salicaceae) and their hybrids 2.     

Leaf litter quality affects aquatic insect emergence: contrasting patterns from two foundation trees

Reciprocal subsidies between rivers and terrestrial habitats are common where terrestrial leaf litter provides energy to aquatic invertebrates while emerging aquatic insects provide energy to terrestrial predators (e.g., birds, lizards, spiders). We examined how aquatic insect emergence changed seasonally with litter from two foundation riparian trees, whose litter often dominates riparian streams of the southwestern United States: Fremont (Populus fremontii) and narrowleaf (Populus angustifolia) cottonwood. P. fremontii litter is fast-decomposing and lower in defensive phytochemicals (i.e., condensed tannins, lignin) relative to P. angustifolia. We experimentally manipulated leaf litter from these two species by placing them in leaf enclosures with emergence traps attached in order to determine how leaf type influenced insect emergence. Contrary to our initial predictions, we found that packs with slow-decomposing leaves tended to support more emergent insects relative to packs with fast-decomposing leaves. Three findings emerged. Firstly, abundance (number of emerging insects m^?2 day^?1) was 25 % higher on narrowleaf compared to Fremont leaves for the spring but did not differ in the fall, demonstrating that leaf quality from two dominant trees of the same genus yielded different emergence patterns and that these patterns changed seasonally. Secondly, functional feeding groups of emerging insects differed between treatments and seasons. Specifically, in the spring collector-gatherer abundance and biomass were higher on narrowleaf leaves, whereas collector-filterer abundance and biomass were higher on Fremont leaves. Shredder abundance and biomass were higher on narrowleaf leaves in the fall. Thirdly, diversity (Shannon’s H′) was higher on Fremont leaves in the spring, but no differences were found in the fall, showing that fast-decomposing leaves can support a more diverse, complex emergent insect assemblage during certain times of the year. Collectively, these results challenge the notion that leaf quality is a simple function of decomposition, suggesting instead that aquatic insects benefit differentially from different leaf types, such that some use slow-decomposing litter for habitat and its temporal longevity and others utilize fast-decomposing litter with more immediate nutrient release.     

Walnut twig beetle landing rates differ between host and nonhost hardwood trees under the influence of aggregation pheromone in a northern California riparian forest

1. Host selection behaviour of the walnut twig beetle (WTB) among hardwood trees was investigated in a riparian forest in northern California by monitoring the landing rate of the beetle with sticky traps on branches baited with 3-methyl-2-buten-1-ol, the male-produced aggregation pheromone.
2. The assay was conducted over 7 days (22 May to 29 May 2017) and compared landing rates on branches of six nonhost species paired with northern California black walnut, Juglans hindsii (the host).
3. A total of 2242/1192 WTB were collected on branches of host/nonhost pairs, and more WTB landed on J. hindsii than on nonhosts in 42 of 58 instances. Female landing rate generally exceeded male landing rate, which underscores the influence of the male-produced synthetic pheromone in this system.
4. Landing rates of WTB males, females, and the combined sexes on boxelder, Acer negundo, and valley oak, Quercus lobata, did not differ significantly from the landing rates on J. hindsii, suggesting that these two nonhost riparian hardwoods do not repel WTB (in the context of the aggregation pheromone).
5. Significantly fewer WTB landed on Oregon ash, Fraxinus latifolia, river red gum, Eucalyptus camaldulensis, Fremont cottonwood, Populus fremontii, and red willow, Salix laevigata, than on J. hindsii, which suggests that these four nonhosts may repel one or both sexes of WTB in the context of the aggregation pheromone. Future analysis of the volatiles from these four hardwood species may lead to the discovery of semiochemical repellents for WTB.

     

Effects of parasitism by a mistletoe on the structure and functioning of branches of its host

Infestation of Acacia acuminata by the xylem-tapping mistletoe Amyema preissii invariably results in inhibition of growth, defoliation and eventual death of host branch parts distal to the mistletoe. Branch sectional areas proximal (P) and distal (D) to mistletoes are used to classify stages of parasitism, with P:D area ratios of 5–6 invariably associated with distal branch senescence. As monopolization of the branch proceeds, mistletoe leaf area increases in parallel with declining host foliage area, and the specific hydraulic conductivity of distal host wood declines sharply relative to that of proximal wood, mineral composition and concentrations of nitrogenous solutes in xylem sap are at no stage appreciably different from those of proximal wood. After the demise of the distal branch parts, the transectional area of the host branch stump increases linearly with increasing mistletoe leaf area, the branch area supporting a unit of mistletoe leaf area always being about 3 times greater than that supporting a unit of host foliage area on unparasitized branches. This differential, compounded with high transpiration rates and selective uptake of host xylem solutes by the haustorium, fosters substantial mineral enrichment of the mistletoe relative to its host. The study provides a background for future investigation of possible cellular mechanisms continuously driving structural and functional changes in favour of the mistletoe.     

Common trade‐offs between xylem resistance to cavitation and other physiological traits do not hold among unrelated Populus deltoides ×Populus nigra hybrids

We examined the relationships between xylem resistance to cavitation and 16 structural and functional traits across eight unrelated Populus deltoides×Populus nigra genotypes grown under two contrasting water regimes. The xylem water potential inducing 50% loss of hydraulic conductance (Ψ₅₀) varied from ?1.60 to ?2.40 MPa. Drought‐acclimated trees displayed a safer xylem, although the extent of the response was largely genotype dependant, with Ψ₅₀ being decreased by as far as 0.60 MPa. At the tissue level, there was no clear relationship between xylem safety and either xylem water transport efficiency or xylem biomechanics; the only structural trait to be strongly associated with Ψ₅₀ was the double vessel wall thickness, genotypes exhibiting a thicker double wall being more resistant. At the leaf level, increased cavitation resistance was associated with decreased stomatal conductance, while no relationship could be identified with traits associated with carbon uptake or bulk leaf carbon isotope discrimination, a surrogate of intrinsic water‐use efficiency. At the whole‐plant level, increased safety was associated with higher shoot growth potential under well‐irrigated regime only. We conclude that common trade‐offs between xylem resistance to cavitation and other physiological traits that are observed across species may not necessarily hold true at narrower scales.     

Limits to water transport in Juniperus osteosperma and Pinus edulis: implications for drought tolerance and regulation of transpiration

1. An air-injection method was used to study loss of water transport capacity caused by xylem cavitation in roots and branches of Pinus edulis (Colorado Pinyon) and Juniperus osteosperma (Utah Juniper). These two species characterize the Pinyon–Juniper communities of the high deserts of the western United States. Juniperus osteosperma can grow in drier sites than P. edulis and is considered the more drought tolerant.
2. Juniperus osteosperma was more resistant to xylem cavitation than P. edulis in both branches and roots. Within a species, branches were more resistant to cavitation than roots for P. edulis but no difference was seen between the two organs for J. osteosperma . There was also no difference between juveniles and adults in J. osteosperma ; this comparison was not made for P. edulis .
3. Tracheid diameter was positively correlated with xylem cavitation pressure across roots and stems of both species. This relation suggests a trade-off between xylem conductance and resistance to xylem cavitation in these species.
4. During summer drought, P. edulis maintained higher predawn xylem pressures and showed much greater stomatal restriction of transpiration, consistent with its greater vulnerability to cavitation, than J. osteosperma .
5. These results suggest that the relative drought tolerance of P. edulis and J. osteosperma results in part from difference in their vulnerability to xylem cavitation.